Which one of the following statements regarding water re-absorption in the tubules is True?
## Core Concept
The renal tubules play a crucial role in regulating water reabsorption, which is vital for maintaining proper fluid balance and electrolyte concentrations in the body. Water reabsorption in the tubules is primarily influenced by the presence of aquaporins and the osmotic gradient established by the loop of Henle. The process is tightly regulated by antidiuretic hormone (ADH), also known as vasopressin.
## Why the Correct Answer is Right
The correct answer highlights that the majority of water reabsorption occurs in the **proximal convoluted tubule (PCT)**, with fine-tuning of water reabsorption happening in the collecting ducts under the influence of ADH. About 65-70% of water reabsorption occurs passively in the PCT, following the reabsorption of solutes like sodium, glucose, and amino acids. The descending limb of the loop of Henle is also permeable to water, but the fine control of water reabsorption is exerted in the collecting ducts.
## Why Each Wrong Option is Incorrect
- **Option A:** This option might suggest that most water reabsorption occurs in the loop of Henle, which is incorrect because, although the loop of Henle contributes to establishing the osmotic gradient necessary for water reabsorption, it is not the primary site for water reabsorption.
- **Option B:** This could propose that water reabsorption primarily occurs in the distal convoluted tubule, which is not accurate. While some water reabsorption can occur here, it is not the major site.
- **Option C:** If this option inaccurately describes the mechanism or site of water reabsorption, it would be incorrect based on the physiological processes described.
## Clinical Pearl / High-Yield Fact
A key point to remember is that **ADH (antidiuretic hormone)** plays a critical role in regulating water reabsorption in the collecting ducts. In states of low ADH (e.g., diabetes insipidus), the kidneys are unable to reabsorb water properly, leading to polyuria and polydipsia. Conversely, high levels of ADH promote water retention.
## Correct Answer: D.